Preparation of coated regenerated cellulose film

ABSTRACT

A process for preparing coated regenerated cellulose film which comprises successively softening the regenerated cellulose gelstate web with polyethylene glycol, drying the web with controlled shrinkage, coating the web and humidifying the web while permitting at least about 1/2 percent longitudinal shrinkage. The resulting coated product exhibits improved permanent shrinkage properties.

nited States Patent Edwards et al.

[151 3,656,993 [451 Apr. 18, 1972 [54] PREPARATION OF COATED REGENERATED CELLULOSE FILM [72] Inventors: Bernard S. Edwards; Vernon C. Haskell,

both of Richmond, Va.

[73] Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

[22] Filed: June 4,1970

21 Appl.N0.: 43,511

[52] U.S.Cl ..l17/56,117/63,117/144,

7 117/145 [51] Int. Cl ..B44d U092 [58] Field of Search ..1 17/56, 144, 145, 63

[56] References Cited 1 UNITED STATES PATENTS 3,442,697 5/1969 Kanej ..1 17/56 X 1.277.695 9/1918 Cavanuugh ..1 l7/56 578,714 3/1897 Bennett ..1 17/56 2,182,765 12/1939 Silshy 117/56 2,328,679 9/1943 Rothrock 1 17/145 2,115,132 4/1938 Alles et a1 ..18/57 3,068,529 12/1962 Kane 18/48 3,515,780 6/1970 OConnell et a1 ..117/145 3,565,669 2/1971 McDermott et al. ..1 17/62 Primary Examiner-William D. Martin Assistant Examiner-M. R. Lusignan Attorney-Donald W. Huntley 57 ABSTRACT A process for preparing coated regenerated cellulose film which comprises successively softening the regenerated cellulose gel-state web with polyethylene glycol, drying the web with controlled shrinkage, coating the web and humidifying the web while permitting at least about /2 percent longitudinal shrinkage. The resulting coated product exhibits improved permanent shrinkage properties.

6 Claims, 2 Drawing Figures PATENTEDAPRIBIHYE i M. 335.5; g msL Z Z 2 3 m u m mm w MUM m 0 W m mm QQW o 3 v m N u-h- L/ hiiiih wwmmwm I O O U l w o o o o o ow o O O O NV O O ON II: llw llll llnllll BY QWMU TTORNEY BACKGROUND OF THE INVENTION In the preparation of regenerated cellulose films from the viscose process, the regenerated web is usually passed through a bath of softener or plasticizer to increase the toughness and durability of the film product. In addition, regenerated celluthe monomeric oxide in the presence of a catalyst until a substantial portion of the polymer is formed as described, for example, in Staudinger, Die l-Iochmolekularen Organischen Verbindungen, 1932, page 287 et seq. The structural formula lose film is normally coated to improve such specific performance characteristics as moisture resistance, heat scalability and gas permeability.

While the addition of softeners to the cellulosic web enhances the durability of the finished product, the hygroscopic nature of most softeners promotes shrinkage of the film product with varying humidity. This shrinkage is particularly .undesirable when the film is used as a tight overwrap in packaging applications, since. shrinkage can result in poor package appearance. and, in situations of extreme humidity variation, can cause the film either to split or to crush the package contents.

Previous attempts to reduce the shrinkage of a finished film product have included various combinations of preshrinking and tensioning during the drying of the cellulosic web. For example, in Kane, U.S. Pat. No. 3,068,529, the softened gel regenerated cellulose film begins the drying step with the application of draw to the film followed by relaxationand completion of the drying cycle while applying additional draw. Unfortunately, however, previous attempts have not resulted in a completely satisfactory method for reducing permanent shrinkage in regenerated cellulose films.

SUMMARY OF THE INVENTION The instant invention provides a simple and economical process for the reduction of permanent film shrinkage in coated regenerated cellulose film products.

Specifically, there is provided a process for producing coated regenerated cellulose film which comprises softening a gel-state web of regenerated cellulose with plasticizer consisting essentially of polyethylene glycol polymer having an average molecular weight between about 285 and 4,000; drying the softened gel-state web to a moisture content of about from4 to 8 weight percent ina manner such that the web is maintained in a wrinkle free condition throughout the drying operation while permittingabout from 2 to 5 percent shrinkage; applying a solvent dispersed polymeric coating to at least one surface of the dried web; removing the solvent from the i applied coating; and humidifying the coated web while permitting at least about 0.5 percent longitudinal shrinkage. Brief Description of the Drawings FIG. 1 is a schematic illustration of an apparatus which can be used in the instant invention.

FIG. 2 is a graphical. illustration of the shrinkage characteristics of a coated film prepared according to the instant process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The regenerated cellulose films used in the instant process are prepared by the viscose process generally described in Brandenberger, U.S. Pats. No. 1,548,864 and No. 1,601,289, both hereby incorporated by reference. In this process, an alkaline aqueous solution of sodium cellulose xanthate is forced through an elongated orifice in the form of a continuous sheet into a coagulated bath where a coherent web is formed and subsequently regenerated, washed, desulfured and bleached. At this point, the gel-state web is in a highly swollen state and contains about from 200 to 350 percent water based on the cellulose content of the web.

In accordance with the instant process, the swollen gel-state web is softened by passing through a tank containing an aqueous solution of plasticizer consisting essentially of polyethylene glycol. The polyethylene glycol should have an average molecular weight of about from 285 to 4,000. The polyethylene glycol can be prepared, for example, by heating of the polyethylene glycol used in the instant invention is as The plasticizer used in the aqueous softener solution should consist essentially of polyethylene glycol. It has been found that this polymer gives unique combination of increased durability while imparting a minimal increase in permanent shrink characteristics. While minor quantities of other softeners such as glycerin and propylene glycol can be used in the softener solution, any such other components should not exceed 15 to 20 percent of the total quantity of plasticizer. While the concentration of the plasticizer in the aqueous solution is not a critical element of the invention, it has been generally found convenient to provide about from 5 to 10 percent by weight of plasticizer in the aqueous solution. Softener bath temperatures of about 70 C. have been found satisfactory for most regenerated cellulose webs.

As the web passes through the softener solution, the aqueous solution replaces or adds to the water content in the swollen web. Accordingly, the web, as it leaves the softener bath, is saturated with an aqueous solution of about 2.0 to 3.5 times the dry weight of cellulose in the web. It is customary to dry this web under controlled moisture removal conditions by passing it through a film dryer whereby the saturated web is placed in intimate contact with a series of specially surfaced heated rolls.

In the drying operation, the moisture content of the web is generally reduced to about from 4 percent to 8 percent, based on the weight of the dry cellulose in the web. During this moisture removal process the web tends to shrink in all directions. Since unrestrained shrinkage results in severe puckers and wrinkles, the shrinkage of the film is restrained by subjecting it to varying degrees of tension as it passes over a succession of heated rotating idler and driven rolls. The dryer rolls in the dryer sections are preferably uniformly heated, for example, using hot water, steam at atmospheric pressure and pressurized steam. The operating temperature ranges for the various sections can vary about from 70 C. to C. in the several zones in the dryer. The shrinkage on the transverse direction is restrained by specially finishing the drying rolls to attain the desired coefficient of friction and by accurately spacing the rolls. The overall longitudinal shrinkage in the drying operation should be about from 2 to 5 percent. Preferably, the softened web is permitted to shrink up to about 3 percent in the longitudinal direction before about 60 percent of the water content of the web is removed. The particular apparatus used for the control of shrinkage during the drying operation can be of the type described in Alles et al., US Pat. No. 2,l 15,132, hereby incorporated by reference.

After drying the softened web, the web has undergone a shrinkage of about from 2 to 5 percent. The dried web is then coated on at least one side with a transparent organic polymeric material of the type generally used in the art to improve moisture resistance, heat sealability and gas permeability. Polymeric materials of this type include nitrocellulose and vinylidene chloride polymers and copolymers. These materials are generally applied as solutions or dispersions in volatile organic solvents such as acetone, methyl ethyl ketone, ethyl acetate, tetrahydrofuran and toluene. The solutions can be applied by any convenient apparatus of the type generally used in the art including immersion coating techniques and contact coating apparatus such as doctor rolls. In general, immersion coating techniques are preferred since it is generally desired to apply the coating material to both sides of the regenerated cellulose web. After coating, the web is dried to remove the solvent and solidify the coating material. The surface can be dried using heated air or other drying techniques including radiant or conduction heating. The coated film is dried under tension to affect little or no dimensional change in the coated material during the coating drying operation.

Since moisture is lost from the regenerated cellulose substrate during the drying operation, the dried coated film is humidified by passing the film through an atmosphere having a high wet bulb temperature. The humidifying atmosphere preferably has a dry bulb temperature of about from 80 to 90 C. and a web bulb temperature of about from 75 to 85 C. The elevated wet bulb temperature can be maintained, for example, by passing a heated air stream through a hot water spray and then through live steam. During the humidifying operation, the web is allowed to shrink or relax at least about 0.5 percent in the longitudinal direction. Preferably a longitudinal relaxation of 0.5 to 1.5 percent is permitted. The relaxation during the humidification can be controlled by supporting the coated dried film between two sets of driven nip rolls, the second set of nip rolls being driven at a speed less than the first set.

The instant invention will be further understood by reference to FIG. 1 which illustrates a representative apparatus which can be used in the instant process. In that apparatus, gel-state web 1 is passed into softener tank 2 containing plasticizer solution 3. The web is passed between upper tank rolls 4 and submerged lower tank rolls 5.

On emerging from the softener tank, the softened web is passed between scraping devices 6 and 7 and then into dryer 8. The dryer comprises a group of heated lower driven rolls 9, a group of upper idler rolls 10 and a vapor-removal system having a lower plenum system 11 and an upper plenum system 12. The rolls 13 of the first stage of the dryer are operated at a peripheral speed greater than the peripheral speed of the upper tank rolls of the softener tank by an amount sufficient to subject the gel-state softened web to a slight draw, for example, of about from 0.1 percent to 0.5 percent.

As the moisture content of the web is reduced to approximately 90 percent of its moisture content as the web entered the dryer, the web enters relaxation zone 14. Included in that zone are several rolls driven at gradually decreasing surface speeds. The surface speed variation of each set of driven rolls is about from 0.1 percent to 1.5 percent in a successive speed decreasing pattern.

The overall moisture content of the web as it leaves the relaxation zone 14 is generally about from 10 percent to 15 percent of the moisture content of the web as it entered the dryer. The relaxed and partially dried web is then passed to the tensioning zone 15. The dryer rolls in this zone are driven at gradually increasing peripheral speeds. Accordingly, the first driven roll in the tensioning zone can have a speed equal to or slightly greater than the peripheral speed of the last roll in the relaxation zone. The final roll in this zone preferably has a peripheral speed of about from 0.4 percent to 1.2 percent greater than the peripheral speed of the first roll in the tensioning zone. The tensioning zone imparts a degree of draw to the web being dried to control the final sheet flatness. The moisture content of the web is reduced in this zone to about 5 percent to 10 percent of the moisture content of the web as it entered the dryer.

The web drying process is completed as the web passes through the heat-setting zone 16 comprising the remaining rolls in the dryer. The peripheral speed of the rolls in this zone is substantially the same as that of the last roll in the tensioning zone. This permits neither drawing or relaxation of the web as its moisture content is reduced to the final desired level of from 4.0 percent to about 8.0 percent based on the cellulose content of the dried web. This zone heat sets the web as it controls the final moisture content therein. The dried and softened web can then be wound onto a core for storage or immediately conveyed to coating tower 17 where it is coated.

The dried and softened web enters the coating tower through opening 18. The web passes through the coating solution bath 19, under the dip tank roll 20, through the doctor rolls 21, through the coating solution smoothing rolls 22 and then into the coating solution drying section 23.

Hot dry air is circulated through the drying section via entrance port 24 and exhaust port 25. This circulated hot air vaporizes the solvent from the coated web and conveys the solvent vapors out of the drying section through the exhaust port.

The coated film, after drying, passes into head roll compartment 26, over head roll 27 and into the humidification and relaxation section 28. Moisture lost in the web during the coating and drying operation is replaced in the humidification and relaxation section by passing humidified air countercurrent to'the web through the humidification and relaxation section through entrance duct 29 and out exhaust duct 30 as the coated web passes through this section. The coated web leaves the humidification and relaxation section by passing under roll 31, around portions of the chill rolls 32 and 33 and onto windup core 34 which is operatively positioned on multiple turret windup 35. The peripheral speed of the chill rolls 32 and 33 I are controlled at 0.5 percent to 1.5 percent less than the peripheral speed of the head roll 27. The coated web is accordingly simultaneously humidified and relaxed. The time of exposure of the web to the humidified air so as to effect the 0.5 percent to 1.5 percent relaxation is dependent upon the thickness of the web and the type of coating material on the web.

The process of the instant invention provides a method for the preparation and coating of regenerated cellulose films which results in coated products having a substantially lower permanent shrinkage than most commercially available films of this type. This reduced permanent shrinkage is believed to be a function of three important parameters of the instant process, namely, the polyethylene glycol softener used in the preparation of the film as well as the relatively high degree of relaxation during the drying process together with the relaxation during the humidification after coating.

The instant invention is further illustrated by the following specific example.

EXAMPLE A coated regenerated cellulose film is prepared using an apparatus similar to that illustrated in FIG. 1. A gel-state web is passed through a 6.5 percent aqueous solution of polyethylene glycol and thereafter dried to a softener content of 13 weight percent polyethylene glycol and a moisture content of about 8 weight percent. The softened film is permitted to shrink about 2.5 in the longitudinal direction during drying. The film is coated with a nitrocellulose coating solution, dried, and then permitted to relax 0.6 percent in a humidified atmosphere.

Film samples for testing are prepared by cutting 4 inch X 6 inch sections from the finished film, the 6 inch dimensions extending in the films machine direction. Each sample is measured to the nearest 0.01 inch and suspended with a clip inside a temperature and humidity controlled oven. The film samples are subjected to an atmosphere of percent relative humidi- I ty at F. for 72 hours, after which the product is subjected to a dry atmosphere of 20 percent relative humidity at 100 F. for 72 hours.

The test samples are measured again to the nearest 0.01 inch and the change in dimension is used to calculate the percent shrinkage. The percent shrinkage in the machine direction for the film samples is plotted in FIG. 2. The average shrinkage for the test film is 1.68 percent with a standard deviation of 0.29. If packages with an average looseness of 2 percent were submitted to the drastic cyclic conditions set forth above, approximately 84 percent of the packages wrapped in the regenerated cellulose film of the instant invention would show no package deformation at all and the other 16 percent would exhibit slight crushing. By comparison, a coated film having an average shrinkage of 2.9 percent would exhibit only 2.5 percent non-crushed packages.

We claim:

1. A process for producing coated regenerated cellulose film which comprises softening a gel-state web of regenerated cellulose with plasticizer consisting essentially of polyethylene glycol polymer having an average molecular weight between about 285 and 4,000; drying the softened gel state web to a moisture-content of about from 4 to 8 weight percent in a manner such that the web is maintained in a wrinkle free condition throughout the drying operation while permitting about from 2 to 5 percent shrinkage; applying a solvent dispersed polymeric coating to at least one surface of the dried web; removing the solvent from the applied coating; and humidifying the coated web while permitting at least about 0.5 percent longitudinal shrinkage.

2. A process of claim 1 wherein the coated web is permitted to shrink about from 0.5 to 1.5 percent in the longitudinal direction while humidifying the coated web.

3. A process of claim 2 wherein the coated web is humidified by passing the web through an atmosphere having a dry bulb temperature of about from 80 to 90 -C. and a wet bulb temperature of about from 75 to 85 C.

4. A process of claim 1 wherein the gel-state web is softened by passing through an aqueous solution of about from 5 to 10 percent plasticizer.

5. A process of claim 4 wherein the plasticizer consists of polyethylene glycol and up to about 20' percent softener selected from glycerin and polypropylene glycol.

6. A process of claim 1 wherein the gel-state web is dried in a manner such that the web is permitted to shrink up to about 3 percent in the longitudinal direction before about 60 percent of the water content of the gel-state web is removed. 

2. A process of claim 1 wherein the coated web is permitted to shrink about from 0.5 to 1.5 percent in the longitudinal direction while humidifying the coated web.
 3. A process of claim 2 wherein the coated web is humidified by passing the web through an atmosphere having a dry bulb temperature of about from 80* to 90* C. and a wet bulb temperature of about from 75* to 85* C.
 4. A process of claim 1 wherein the gel-state web is softened by passing through an aqueous solution of about from 5 to 10 percent plasticizer.
 5. A process of claim 4 wherein the plasticizer consists of polyethylene glycol and up to about 20 percent softener selected from glycerin and polypropylene glycol.
 6. A process of claim 1 wherein the gel-state web is dried in a manner such that the web is permitted to shrink up to about 3 percent in the longitudinal direction before about 60 percent of the water content of the gel-state web is removed. 